Cylindrical forged tungsten electrical contact



1963 w. G. CHISWELL 0 CYLINDRICAL FORGED TUNGSTEN ELECTRICAL CONTACTAug. 27, 1957 n 19 i INN I" n 16 4/. 20 Ill 21 19 V 23 @f' -2 24 I W 11/f9 11 30 22 31 L 30 31 s 4 3,106,628 CYLINDRICAL FORGEB TUNGSTENELECTRICAL CONTACT William G. Chiswell, Waukegan, lll., assignor toFansteel Metallurgical Corporation, a corporation of New York Filed Aug.27, 1957, Ser. No. 680,452 2 Claims. (Cl. 200-166) The present inventionrelates to electrical contacts and a method of making electricalcontacts. It particularly relates to electrical contacts and a method ofmaking electrical contacts of the type used in devices such as relaysand voltage regulators, etc.

The electrical contacts used in devices such as relays and voltageregulators must carry high currents and are subjected to high currentdensities. They are also subjected to frequent circuit opening andclosing operation. As a result, these contacts must withstand hightemperature, due to arcing, and must have a high wear resistance becauseof the frequency with which they are opened and closed. The contactsalso must have a surface configuration that will resist wear and thataids in breaking arcs that occur between adjacent contacts.

For these purposes, contacts of this type have been made of materialsuch as tungsten or other similar metals having high melting points.Although lower melting point metals have been used, tungsten is the mostcommonly used material. The present invention is not, however, concernedwith the particular material used for makin the contact.

To reduce arcing and to reduce the effects of mechanical wear, it hasbeen the practice to make contacts of this type with a crowned orsegmental spherical face which may or may not, as desired, having aslight fiat portion at the center of the crown.

Crowning of these contacts is important to the wear and are resistancecharacteristics. However, crowning has been a costly and time-consumingoperation since it is generally accomplished by hand grinding. The handgrinding is not only inefiicient, but subject to inaccuracies. Thegrinding of the crowned faces of the contacts also leaves grindingstreaks or marks on the face of the contact. These grinding streaksreduce the efficiency of the contact by giving the face a rough andirregular surface and by promoting oxidation and increasing the surfaceresistance of the contact.

By the present invention these difficulties are overcome and face of thecontact is provided with a smooth crowned configuration that issegmentally spherical. The contacts of this invention are also given aconcave back face which permits more efiicient and easier mountingthereof on the backing and supporting member to which it is brazed.

In accordance with this invention contacts are made by cuttingcylindrical segments from a rod made of contact material, such astungsten. The pieces are cut to have an appropriate contact thickness.As an example, a tungsten rod is cut in a diamond disc cutoff machinethat operates automatically to cut off disc blanks at a thickness ofabout 0.030 inch. The rod has a diameter of about 0.147 inch. Contactsmade from these discs may be used in an automotive voltage regulator.The disc blanks are then placed in a forging machine, on anappropriately configurated die of a diameter of about inch with aconcave die face having a diameter of 0.180 inch and a 0.700 inch radiusof curvature.

With tungsten, the disc blank is then heated by such means as aninduction heater or an cry-acetylene torch to a temperature of fromabout 1600 F. to about 1800 F., substantially below the melting point ofthe metal but high enough to permit forge shaping and crowning of themetal disc. The disc is then struck by a male die operated Patented Oct.8, 1963 from a pneumatic or hydraulic ram. The ram is pneumatically orhydraulically operated so that it may cornpe-nsat-e for slightvariations in thickness of the disc.

The male die, in this example, has a diameter of about 0.125 inch and isprovided with a segmental spherical face having a radius of about 0.670inch.

After the disc is struck, it is ejected from the forging machine by anair blast. The slight resilience of the disc is sufiicient to providethe disc with a final outer surface radius of from about 0.750 inch toabout 0.800 inch, and a substantially identical inner concave surfaceradius. The external surface of the disc is smooth since it was forcedagainst a highly polished die surface.

The advantages and features of the invention, the details thereof andvariations therein will become clear to those skilled in the art andothers from the following detailed description of the invention and anembodiment thereof, from the claims, and from the accompanying drawingsin which each and every detail shown is included as a part of thisspecification, in which like reference numerals refer to like parts, andin which:

FIGURE 1 is a fragmental partly sectional view of a die portion of aforging machine with a contact disc therein;

FIGURE 2 is a sectional view of a contact disc embody ing this inventionand mounted on a backing and mounting pin;

FIGURE 3 is an elevation view of a contact disc blank prior to beingforge crowned;

FIGURE 4 is a vertical sectional view of a forge crowned contact discembodying this invention and made by the method of this invention;

FIGURE 5 is a plan view of the mounting face of the contact disc ofFIGURE 4; and 0 FIGURE 6 is a plan view of the contact face of thecontact disc of FIGURE 4.

Contact discs embodying this invention are cut in any convenient mannerfrom rods of contact material, such as rods of tungsten. The manner ofcutting the discs is not critical to this invention; however, the discsmay be cut manually, semiautomatically, or automatically byappropriartely jigged mechanisms. The discs'cut from the rods areslightly smaller in diameter and slightly smaller in thickness than thefinal contact discs.

After the disc blanks are cut from the rod they are fed, in anyconvenient manner, manually or automatically, into a forging or modifiedpunching machine shown in part in FIGURE 1. Therein it is shown that themachine has a base 12 appropriately recessed, as at 13, to receive afemale die 14 on which the disc blank is placed. The die 14 has aspherically curved and polished concave face 15, the radius of curvatureof which is appropriate to allow for spring-back of the disc materialafter forg-. ing so that the disc will have the desired spherical face.In one example of such a die and disc, if the disc is to have a radiusof curvature of from about 0.750 inch to about 0.800 inch, and is to bemade of tungsten, with the forging operation being effected at atemperature of from about 1600' F. to about 1800 F., then the radius ofcurvature of the face 15 on the die 1-4 will be about 0.700 inch. Thedisc 11 is held coaxially over the face of the die 14 by any convenientchuck arrangement such as the resilient holding fingers 16. Thesefingers hold the disc in place for heating and forging.

The heating of the discs is effected in any convenient manner such as byan induction heater 17 disposed above the fingers 16 and about the disc11. It is also suificiently spaced from the die 14 and supporting block12 to permit the finished disc 11 to be air blast ejected from themechanism. Further, the induction heater 17 shown in FIGURE 1 is asingle-turn heater with a sufiiciently large \J inner diameter to permitthe upper movable punch or die 18 to pass therethrough without engagingthe heater.

When the heater 17 is energized, the disc 11 will be heated to asufficient temperature to permit forging. Then a pneumatic or hydraulicram 19 will drive the punch or male die 18 downwardly to strike theupper surface of the disc blank. The bottom face 20' of the die 18 isprovided with a segmental spherical configuration which, in thisexample, has a spherical radius of about 0.670 inch. The diameter of thedie 18, in this example, is about 0.125 inch.

After the blank is struck by the male die it is transformed from theconfiguration of blank shown in FIGURE 3 to the configuration of discshown in FIG- URE 4. It is then ejected from the machine by an air blastemanating from a nozzle 21 connected to any appropriate source of airand controlled in any convenient manner so that air will blast from thenozzle 21 after the die 18 is retracted by the ram 19.

The disc expands slightly under the force of the male die striking thedisc so that it is enlarged slightly in diameter and is enlargedslightly in rim thickness. This latter effect is due to the fact thatthe male die is made slightly smaller in diameter than the diameter ofthe disc. The effect of using a smaller diameter die than disc alsoprovides the disc with a fiat rim 22 about the face of the disc that isstruck by the male die and provided with a concave central sphericalcurved portion 23.

The face 24 of the disc 11, which face was in contact with the face ofthe female die 14, is now convexly curved with a spherical radius thatis substantially the same as the radius of the concave face 23. Thiscurvature has a slightly greater radius than the radius of the dies dueto resilience of the material.

However, both the concave and the convex face are substantiallyperfectly formed and smooth since they were formed against highlypolished dies. They do not have grinder marks or other similar marksthereon such as might occur in hand grinding and the contacts areuniformly made so that they are freely substitutable andinterchangeable.

The fiat rim 22 about the concave curved portion of the disc 11 providesan alignment aid and brazing aid for mounting of the disc on a contactbacking pin. The relation between the concave face of the disc and amounting pin is shown in detail in FIGURE 2 wherein a complete assemblyis shovsm in section.

The contact mounting pin 25 has a mounting shank 26 for mounting theassembly on a relay arm. To position the shank on the relay arm, theshank carries a radial shoulder 27 that may be hexagonally contiguratedto be held by a wrench or other appropriate mounting tool. Beyond theshoulder portion 27, the mounting pin has a head 28 onto which the disc11 is secured coaxially with the pin.

The face 29 of the head 28 is spherically curved to a radius slightlygreater than the radius of spherical curvature of the concave face 23and the convex face 24 on the disc 11. This provides a slightly greaterbrazing gap at the center of the disc that at the inner edge 30 of therim 22 on the disc 11. In the example given the radius of curvature ofthe face 29 of the head is about 0.900 inch.

By providing a centrally enlarged gap between the disc and the pin head28 and a radially expanding gap between the rim 22 and the face 29 ofthe head, more etlicient brazing is facilitated and the members permitbrazing material such as is shown at 31 to fill the gap and be acceptedbetween the two parts to hold the parts together. Without this enlargedgap the brazing material would be squeezed out of various areas betweenthe disc and the pin so that voids would occur in the brazing area. Suchvoids are highly undesirable since they reduce the electrical contactingefficiency and increase the contact resistance of the members.

Devices embodying this invention provide efiicient contacts that havelong life in operation as well as on the shelf. Since there are noirregularities on the actual contact face 24 of the disc 11, oxidationof the face is minimized and sparking and arcing are substantiallyreduced.

From the foregoing it will be seen that numerous modifications andvariations may be effected without departing from the true spirit andscope of the novel concepts and principles of this invention.

1 claim:

1. An electrical contact comprising a substantially cylindrical tungstendisc formed of a homogeneous metallic composition and having an annularouter surface and a pair of opposed transverse surfaces, the opposedsurfaces of said disc being smooth to increase the efficiency of thedisc as a contact member and being substantially spherically curved, oneof said opposed surfaces being convex to reduce arcing and to reduce theeffects of mechanical wear, and the other of said opposed surfaces beingconcave, said opposed surfaces having substantially the same radius ofcurvature, the curvature of said concave surface terminated inwardlyfrom the periphery of the disc whereby the disc is provided with a flatrim between the concave curved portion of said other surface and theouter periphery of the disc.

2. An electrical contact comprising a substantially cylindrical forgedtungsten metal disc of unitary structure, the opposed faces of said discbeing spherically curved, one of said faces being convex and the otherof said faces being concave, the curvature of said concave faceterminating inwardly from the periphery of the disc whereby the disc isprovided with a fiat rim between the concave curved portion of saidother face and the outer periphery of the disc.

References Cited in the tile of this patent UNITED STATES PATENTS1,046,823 McBerty Dec. 10, 1912 1,220,886 Pfanstiehl Mar. 27, 19172,181,083 Fayette Nov. 21, 1939 2,439,570 Hensel et ai. Apr. 13, 19482,468,888 Mekelburg May 3, 1949 2,624,820 Fayette Jan. 6, 1953 2,646,613Enzle r July 28, 1953 2,737,708 Rollfinke Mar. 13, 1956

2. AN ELECTRICAL CONTACT COMPRISING A SUBSTANTIALLY CYLINDRICAL FORGEDTUNGSTEN METAL DISC OF UNITARY STRUCTURE THE OPPOSED FACES OF SAID DISCBEING SPHERICALLY CURVED, ONE OF SAID FACES BEING CONVEX AND THE OTHEROF SAID FACES BEING CONCAVE, THE CURVATURE OF SAID CONCAVE FACETERMINATING INWARDLY FROM THE PERIPHERY OF THE DISC WHEREBY THE DISC ISPROVIDED WITH A FLAT RIM BETWEEN THE CONCAVE CURVED PORTION OF SAIDOTHER FACE AND THE OUTER PERIPHERY OF THE DISC.